ISS 3 ZX81 wont start to K

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mlasham
Posts: 1
Joined: Sat May 20, 2023 4:31 am

ISS 3 ZX81 wont start to K

Post by mlasham »

Hi

New to the forums, been a Sinclair fan ever since I got my first ZX81 way back then.

I have an ISS 3 ZX81 USA version that wont start.
I've replaced all the ICs with known working ones, replaced the two capacitors and two transistors and still get the same fault.
PIN 16 on the ULA gives a garbage signal but the ULA works in other ZX81s

I've looked for bad solder joints, broken tracks, etc but cannot find anything.
Voltages look good.
Clock on ULA pin 14 looks ok

Any suggestions on what else I can try please.
Weirdly a friend of mine also has the same ZX81 board and has a very similar fault.

Thanks
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1024MAK
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Re: ISS 3 ZX81 wont start to K

Post by 1024MAK »

Hello and welcome.

You say it won’t start, is it a black screen, a white screen or something different that you are getting?

You say that you’re replaced all the ICs with known working ones, how do you know that the ICs are working okay? Did they come from another working machine?

If you do have another working machine, did you try the ‘suspect’ chips from the non-working ZX81 in the working machine? Did it still work? If yes, it proves the swapped chip is good.

What type of transistor did you use as replacements? It’s very uncommon for either of the original transistors to fail. But a new replacement transistor may not work correctly in the circuit if it has different characteristics.

Can you post a photo of your ZX81 board please.

What test gear do you have? I’m assuming you have a ‘scope.

My next step would be to measure pin 26 (RESET) on the Z80 to see if the voltage has gone high. Then short out the two legs of C5 (or short Z80 pin 26 to any suitable 0V/GND point) and observe the voltage on your ‘scope as the capacitor quickly charges up.

Then test pin 6 on the Z80 looking for the 3.25MHz clock signal. It must switch between the +5V voltage and 0V for the Z80 to work correctly.

If everything is okay so far, then test the following pins:

Pin Function
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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